1. Field of the Invention
The invention is based on a pressure booster for a fuel injection system for internal combustion engines; the pressure booster has a stepped piston which is displaceable in a bore and whose end faces each define one pressure chamber, and a first, larger end face of the stepped piston defines a first pressure chamber, which can be made to communicate with a high-pressure reservoir, and a second, opposed, smaller end face of the stepped piston defines a second pressure chamber, communicating with an injection nozzle; and the cross-sectional change of the stepped piston and a shoulder in a housing of the pressure booster define a relief chamber.
2. Description of the Prior Art
In such pressure boosters, refilling of the second pressure chamber and of the injection nozzle has been done in the past by a restoring spring acting on the stepped piston. A disadvantage of this prior art version is that the restoring spring must have a high spring rate, in order to enable the refilling in a sufficiently short time between injections. The result is large dimensions of the restoring spring. In some pressure boosters, especially those with a low boosting ratio, the requisite installation space for such a restoring spring is often unavailable.
It is the primary object of the invention to furnish a pressure booster whose demand for space for the means for refilling the pressure booster are low and which is simple in construction.
According to the invention, this object is attained by a pressure booster for a fuel injection system for internal combustion engines, in which the pressure booster has a stepped piston which is displaceable in a bore and whose end faces each define one pressure chamber; in which a first, larger end face of the stepped piston defines a first pressure chamber, which can be made to communicate with a high-pressure reservoir, and a second, opposed, smaller end face of the stepped piston defines a second pressure chamber, communicating with an injection nozzle; in which the cross-sectional change of the stepped piston and a shoulder in a housing of the pressure booster define a relief chamber; and in which the first pressure chamber and the relief chamber can be made to communicate hydraulically with one another.
This pressure booster according to the invention has the advantage that the pressure present in the first pressure chamber after the termination of the injection can be utilized to generate a hydraulic force, acting on the cross-sectional change of the stepped piston in the relief chamber, that returns the stepped piston to its outset position before the onset of the injection. At the same time, the pressure in the second pressure chamber of the pressure booster collapses, so that the injection is ended. As a result, the refilling of the second pressure chamber in the injection nozzle is made possible or is facilitated substantially. Furthermore, because it is possible to dispense with a large-sized restoring spring, the relief chamber can be kept small, which is necessary especially in pressure boosters with a low boosting ratio.
A variant of the invention provides that between the high-pressure reservoir, first pressure chamber and relief chamber, a control valve, in particular a 3/2-way control valve, is disposed, which hydraulically disconnects the high-pressure reservoir from the relief chamber or the high-pressure reservoir from the relief chamber and the first pressure chamber, so that all the functions of the pressure booster can be controlled simply and reliably.
One version of the invention provides that the control valve is actuated by a piezoelectric actuator or an electromagnet, so that depending on the operating conditions and the requisite control forces, a suitable actuator can be employed.
In one feature of the invention, a restoring spring is fastened into the relief chamber and is braced on a stationary support and in the process urges the stepped piston, at its cross-sectional change toward the relief chamber, counter to its pumping direction, so that the refilling of the pressure booster of the invention is reinforced by the restoring spring, and the best possible utilization is made of the existing relief chamber. Furthermore, the restoring spring has the task, if pressure is absent in the high-pressure reservoir of the fuel injection system, for example because the engine has been shut off, of moving the stepped piston into its outset position.
In a further embodiment of the invention, a leaking oil return of the pressure booster feeds fuel to the injection nozzle via a check valve, so that as soon as the pressure in the second pressure chamber drops below the pressure of the leaking oil return, the pressure booster is refilled without additional pumping means. The check valve prevents the return flow of fuel from the second pressure chamber or from the injection nozzle into the leaking oil return.
In another feature of the invention, the check valve discharges into the second pressure chamber, so that the fuel in the high-pressure region of the fuel injection system is exchanged continuously, namely from the second pressure chamber via a high-pressure path and to the injection nozzle. As a result, the fuel in the high-pressure region remains relatively cool, which improves the operating performance of the fuel injection system.
In a further feature of the invention, the check valve is spring-loaded, so that a more-stable operating performance of the check valve is achieved.
Another variant provides a stroke stop, which limits the motion of the stepped piston into the first pressure chamber, so that the first pressure chamber always has a certain minimum volume and thus a certain xe2x80x9csoftnessxe2x80x9d. This reduces force peaks and lessens the load on the individual components.
In another version it is provided that the stroke stop is disposed in the first pressure chamber, resulting in a simple, compact design, and furthermore the stroke stop transmits essentially only forces in the axial direction of the stepped piston to this stepped piston.